Moog Lab Series L7 reverb not working. I'm stuck.

[ServiceBob]

Hi!
I am currently repairing a Lab Series L7 amplifier for a friend. The amp seems to be functioning well, with the exception of the reverb, which is not working. I have checked the reverb tank, and it appears to be in good condition. Furthermore, the return circuit seems to be functioning properly. If you turn up the Reverb control and tap the tank, you can hear the springs bouncing around just fine. I have also swapped in a known working tank, but the issue persists. I suspect that the problem lies in the reverb driver. However, my expertise lies more in tubes than solid-state electronics, and I'm unsure about what to do next.

I have attached the relevant section of the schematic that I believe is related to the issue. The voltages on the schematic match the ones I have measured, except for the 1.5v measurement in the lower left corner, which I have measured to be about 9.6v instead. I have used a scope to trace the signal and found that it is present (blue) at the inverting input of the op-amp. However, the output (pin 6) is a steady 12v DC.

To address the issue, I have replaced all of the components highlighted in yellow. I had planned to replace the electrolytic components anyway since they are old, and the two 2200/35 caps measured half their rated values when tested. If you require any additional voltage measurements to aid in diagnosis, please let me know, and I'll be happy to provide them.
The whole schematic is available at http://www.rru.com/~meo/Guitar/Amps/Lab ... scheme.pdf
Thanks in advance for your help.

[bepone]

for the start measure all those resistors if they have the value..

[Stevem]

So is the plus and minus 15 volts present on that 741 chip?

If it does not have both and if they are not close enough to being the same then the chip will not function and just pass whichever voltage is higher to its output pin.

[martin manning]

The voltages on the schematic match the ones I have measured, except for the 1.5v measurement in the lower left corner, which I have measured to be about 9.6v instead. I have used a scope to trace the signal and found that it is present (blue) at the inverting input of the op-amp. However, the output (pin 6) is a steady 12v DC.

What about the [<80mV} at the junction of the 4.7 ohm resistors? I'd think that would be off, and that is likely driving the 741 output to 12V. I suspect the output transistors Q202/Q204. Pull them out and test them.

[ServiceBob]

Thanks for the replies.

Yes, the voltage was off at the junction of the two 4.7Ω resistors. And it was Q202 that was deficient. My buddy had bought the replacement transistors from a seller on eBay, and dumb me didn't test them before installing them. Of course, according to Mouser, the TIP29 and TIP30 are OBSOLETE, so I overnighted (he needs the amp tomorrow night) a TIP41/TIP42 combo and (after testing them ) installed them. Problem solved.
Thanks again!

[martin manning]

Good to hear it's sorted, thanks for the report. eBay parts are always suspect, IMO.